This invention relates, firstly, to a process for reducing iron ores and more particularly to a process for producing reduced or sponge iron by reducing iron ore with carbon which has been produced as a by-product of thermal cracking of heavy oil as a reducing agent.
As is known, the blast furnace-converter process is being widely utilized at present as a process for producing steel. This blast furnace-converter process is generally considered to be a substantially perfected technique, but this does not mean that it is not accompanied by any problems. More specifically, in this blast furnace-converter process, in order to produce iron by removing oxygen (reducing) from iron ore (iron oxide), pig iron containing an excess of carbon is once obtained in the blast furnace and then, in the converter, the excess carbon, together with the accompanying silicon, phosphorous, etc., is oxidized and removed thereby to produce steel. Because of this procedure, the efficiency is poor in theory. Moreover, there is a problem in that strongly caking coal used as a coke raw material for blast furnaces is becoming scarce throughout the world.
Another process which comprises carrying out reduction of iron ore in the solid state while it is in contact with a reducing agent, in general, to a reduction degree of 85 percent or more thereby to obtain sponge iron, and melting and refining this sponge iron in an electric furnace thereby to produce steel has been developed. This process does not entail the theoretically wasteful combination of superfluous reduction followed by oxidation of excessive carbon and the accompanying silicon, phosphorous, etc., which is required in the above described blast furnace-converter process. Another advantageous feature is that strongly caking coal, which is a coke raw material for blast furnaces, is not required.
Furthermore, a technique wherein the thus reduced iron ore pellets of reduction degree of 70 to 90 percent are used as starting materials for blast furnaces in order to effect a saving in the consumption of fuel coke and to improve productivity is being industrially tested.
The reason why the use of these solid state or direct reduction processes do not become widespread throughout the world is that the regions where the reducing agents and the raw materials thereof used in the production of sponge iron, that is, coals, brown coal, natural gases, as well as sources of carbon, H.sub.2, and CO gas, etc., are unevenly distributed throughout the world, and regions where such processes become economically advantageous are limited.
The processes for producing sponge iron are classified by (a) the type of reducing furnace used into the rotary-kiln process, the shaft furnace process, the fixed-bed furnace process, and the fluidized-bed furnace process and by (b) the kind of reducing agent into the solid reducing agent process and the gaseous reducing agent process. In general, the rotary-kiln process is used with the solid reducing agent process, while the remaining three processes divided by type of furnace are used with the gaseous reducing agent process.
Leaving aside, for the moment, the production of sponge iron, the background of the second aspect of this invention will be considered. In view of the limited reserves of petroleum resources, the conversion of relatively heavy fractions in the petroleum fractions into light fractions thereby to increase their commercial value is an important problem. For this purpose, the fluidized catalytic cracking process (FCC process) wherein heavy oil is subjected to catalytic thermal cracking in the presence of catalyst particles such as silica or alumina in a fluidized state has been used from the past.
In this FCC process, however, carbon (coke) produced as a by-product in the thermal cracking of a heavy oil is deposited on the catalyst particles and lowers their activity. For this reason, frequent regeneration of the catalyst is necessary. Furthermore, another difficulty is that this process is applicable to only distillate oils such as ordinary gas oil and high-quality residue oils of limited kinds.
The fluid coking process of recovering as a product the by-product coke in the above described thermal cracking of heavy oil is also widely practiced. In this process, heavy oil is thermally cracked with the use of fine coke in fluid state as a heat and fluid medium. Because the fine coke is used, not as a catalyst, but merely as a heat and fluid medium, there is no problem of loss in activity even when the by-product coke is deposited. Accordingly, this process has the advantage of ease of processing the heavy oil and is generally used for the preparation of the feed oil of the FCC method.
The coke produced as a by-product in the fluid coking process is taken out of the reactor, and a portion thereof is burned to provide heat for heating fine coke which is recirculated into the reactor. At the same time, the remainder of the coke is taken out as a product. In contrast with the delayed coking process which is comparable as a method of processing heavy oil, this fluid coking process is a fully continuous process. While this process has a number of advantages such as high yield of cracked products, the quality of the product coke is so poor that it cannot be used except as a fuel.